1. Field of the Invention
The present invention relates to a boil-off gas processing system for burning and discharging a boil-off gas produced from liquid hydrogen, and in particular, to a system built into a vehicle, where the vehicle is driven by a device using hydrogen, such as a hydrogen fueled engine or a fuel cell.
2. Description of the Related Art
In an example structure of conventional fuel cells, a solid polymer electrolyte membrane is placed between an anode and a cathode, and a hydrogen gas functioning as a fuel gas is supplied to the anode while air or the like which functions as an oxidizing gas is supplied to the cathode. The chemical energy produced by the oxidation reduction reaction of these gases is directly extracted as electrical energy. Such a solid polymer electrolyte membrane fuel cell has a high energy-output efficiency and is environmentally clean; thus, this type of the fuel cell is often employed in motor vehicles or the like.
In order to supply hydrogen to such a fuel cell, (i) pure hydrogen stored in a storage device (a high-pressure hydrogen tank, an alloy tank for hydrogen storage, a liquid hydrogen tank, or the like) may be supplied, or (ii) a hydrocarbon fuel may be reformed by a fuel reformer into a reformed gas including hydrogen, thereby supplying this reformed gas.
As for the former method for supplying pure hydrogen, liquid hydrogen has become the focus of attention and been investigated as a source of hydrogen supply to a vehicle using a fuel cell or hydrogen fueled engine (i.e., a vehicle using hydrogen as a fuel) because of its high energy-storage density and also because liquid hydrogen can be charged into a fuel tank at relatively high speed.
However, liquid hydrogen has an extremely low boiling point of −253° C., and in a tank (built in a vehicle) for storing liquid hydrogen, a boil-off gas is produced when liquid hydrogen is vaporized due to heat from the outside air. The produced boil-off gas causes increase in the pressure in the tank; thus, it is necessary to suitably discharge and process the boil-off gas.
In order to process the boil-off gas, Japanese Unexamined Patent Application, First Publication No. Hei 5-180397 discloses a technique of storing a boil-off gas, which is produced during transportation of liquid hydrogen by using a tanker, in a hydrogen storage alloy which can absorb the boil-off gas. The hydrogen storage alloy has a relatively high storage capability per unit volume and is preferably used for boil-off gas processing systems.
However, if a vehicle has not been used and driven for a long period, a considerable amount of boil-off gas is produced. In order to store all the produced boil-off gas, a very large hydrogen storage alloy tank is necessary, which is not preferable because a boil-off gas processing system built into a vehicle should be small and light.
All of the produced boil-off gas may not be processed by a hydrogen storage alloy or the like. Here, direct discharge of the remaining boil-off gas towards the atmosphere is not preferable, and a technique for burning and discharging the remaining gas by using a catalytic combustor has been investigated.
However, as explained above, the temperature of boil-off gas is very low, and even if such boil-off gas is introduced into a catalytic combustor, the catalyst may not have an temperature necessary for activation. In such a case, no catalytic reaction occurs and the boil-off gas may be discharged without burning.